28 GHz Varactor-Based Reconfigurable Intelligent Surface: Wideband Design and Evaluation of Continuous vs. Quantized Phase Control

This paper presents a wideband varactor-based reconfigurable intelligent surface (RIS) operating around 28 GHz with realistic varactor parasitic modeling. The proposed RIS unit cell achieves continuous phase coverage of up to 344.8 degrees with low reflection loss over a 24.5% bandwidth (24 GHz-30.7 GHz), ensuring compatibility with standard 5G millimeter-wave (mmWave) bands. A 15 × 15 unit-cell RIS is simulated under nearfield excitation, and its beamforming performance is analyzed for continuous as well as 1-bit, 2-bit, and 3-bit phase control schemes. Under oblique incidence, the proposed RIS excited by a standard horn antenna achieves a peak directivity of 23.60 dBi toward broadside using continuous phase control. A comparative investigation reveals that 3-bit quantization offers near-identical performance to continuous control in terms of main beam directivity and sidelobe suppression, with only 0.42 dB degradation. Wide-angle beam steering across from -45 to 45 degrees is also demonstrated over 26 GHz-28 GHz, confirming the design's robustness under oblique incidence and broadband operation.